When an expert has not kept up with his or her field for 20 years, the problem with the testimony is

Question

the expert is biased.

the person is being used as an expert, but is not recognized as an expert.

the expert is in the wrong field of expertise.

the expert is being paid for the testimony.

Answer 1

The most correct answer is "the person is being used as an expert, but is not recognized as an expert."

To be an "Expert" means that you are recognized by your professional peers to be an authority.  Otherwise you are known to be working in that field of expertise.  Of course, some things don't change much over 20 years and a person who works in that feild will know more than the average person.  An example is a plumber may not know about programming the latest timed watering systems but when the toilet is flushed it still flows downhill. -lol